The present invention relates to diagnostic ultrasound systems. In particular, the present invention relates to method and apparatus for generating an ultrasound pulse sequence for approximating desired transmit spectrums.
Ultrasound systems are used to generate diagnostic ultrasound images of many different tissue and systems within a body. However, the ultrasound signal is progressively attenuated as it travels deeper into the tissue. Certain signal waveforms, such as a Gaussian waveform, are desirable as the waveform maintains its spectrum even when experiencing frequency dependent attenuation caused by tissue. For example, echoes from liver tissue will exhibit a depth dependent downshift in the center frequency of the waveform. Depending upon the spectrum of the transmitted pulse, bandwidth may be lost in echoes originating from large depths. It is well known that Gaussian waveforms exhibit no such loss of bandwidth.
In the past, many ultrasound systems have used bipolar transmitters to generate the ultrasound pulses. These transmitters typically generate waveforms defined by a sequence of positive and negative pulses, such as a square wave, with no intermediate zero segments. Bipolar transmitters are inexpensive to make and simple to control, but have limitations in the spectrum of the pulses it can generate. Also pulse width modulation of bipolar waveforms has been used for acoustic power control. This is done by reducing the duration of all of the positive and negative pulses of the base bipolar transmit waveform by the same fraction while the frequency of the waveform is maintained by inserting zero-segments between the pulses. This reduces the amplitude of the transmitted pulse while not substantially changing its spectrum within the pass-band of the transducer. In the past, use of pulse width modulated waveforms of this type has been restricted to Color flow or PW Doppler operation in simultaneous (duplex or triplex) operation with B-mode imaging using regular bi-polar waveforms.
In transmit apodization, the transmit pulse amplitude is usually progressively reduced towards the edges of the array relative to the center of the array. This is done to reduce sidelobes of the transmitted beam. Therefore, progressive pulse width modulation has been used toward the edge of the array to reduce the apparent amplitude of the pulse without substantially changing its spectrum. However, in the past, the spectrum of the transmit apodized waveform was limited to that of conventional bi-polar waveforms (i.e., the waveform of the central part of the array was always a conventional bi-polar pulse with no intermediate zero segments). Thus, many desired signal waveforms could not be successfully approximated.
By way of example only, FIG. 15 illustrates a conventional pulser 170. The pulser 170 has two switches, switch SW11 172 and switch SW21 174. Positive high voltage +V1 is provided to input 176, which is connected to one side of switch SW1 172. Negative high voltage −V1 is provided to input 178, which is connected to one side of switch SW21 174. A resistor 184 connected to ground 186 provides the ability to pull the waveform to ground when neither of the switches 172 and 174 are closed. A controller 180 controls the switches SW11 172 and SW21 174 to produce an output waveform on output 182. Unfortunately, the resistor 184 needs to be small to provide the short transition times required in pulse width modulation, which leads to high losses.
Other ultrasound systems have used a large number of voltage levels to produce pulse sequences approximating arbitrary signal waveforms, such as providing 32 different levels of voltage to approximate, for example, a Gaussian waveform. However, using multiple voltage supplies is expensive to implement. These types of transmitters also have low efficiency, and thus high power consumption. Therefore, the aforementioned systems are expensive and inefficient to implement and maintain.
Thus, a system and method are desired to produce output pulse sequences to approximate desired waveforms that addresses the problems noted above and others previously experienced.